The Effect of Strain Ageing on Plastic Deformation in High Strength Steel

Abstract

The influence of strain ageing on the mechanical properties, plasticity and fracture of quenched and tempered martensitic 300M steel is investigated. Tensile testing with variable strain rate identified the presence of negative strain rate sensitivity at room temperature. The existence of dynamic strain ageing at room temperature was also confirmed for strain rates between 0.00133-0266 s-1 using digital image correlation (DIC). Strain ageing resulted in significant strengthening at room temperature, which masks the presence of dynamic strain ageing upon re-loading. Fracture analysis conducted using scanning electron microscopy (SEM) revealed only minor differences in the failure mechanisms and fracture morphology caused by deformation and ageing. A model by Brown and Embury is used to link inclusion size and distribution to failure strain, and the failure mechanism of 300M was determined to be the formation of micro-voids on transition carbides and inclusion particles. Models for dislocation evolution and locking based on fundamental theories were also produced and successfully used to model the transient strengthening of 300M following ageing at room temperature using differential dislocation evolution equations.